Abstract
Invasive Non-Native Plant species (INNPs) establishment along rivers continues to accelerate globally, with potential consequences for riverbank stability, ecosystem functions and services. Riverbank stability is dependent on the balance between destabilising forces (e.g., shear stress) and stabilising influences (e.g., sediment cohesion and native vegetation reinforcement). However, disturbance driven INNPs invasions are attributed to decreasing riverbank stability through outcompeting and replacing native vegetation. One such INNPs, Impatiens glandulifera (Himalayan Balsam), has been linked to increased riverbank erosion over winter owing to an annual post summer phenological dieback. Yet the impact of I. glandulifera on riverbank stability remains unknown. Here we show that I. glandulifera invasion indirectly amplifies seasonal reductions in riverbank shear strength. We found, using piecewise Structural Equation Modelling, that previous summer I. glandulifera cover indirectly influences riverbank shear strength via suppression of overwinter native vegetation cover, promoting the formation of bare ground. Riverbank shear strength was ~ 30% lower over winter in invaded compared to uninvaded sites. Our results demonstrate how an annual INNPs can amplify expected seasonal decreases in riverbank stability because of a seasonal phenological feedback. INNPs influence on riverbank stability has key implications for ecosystem function and services. As invaded riverbanks may be more vulnerable to erosion leading to morphological changes and downstream habitat degradation from increased sediment loading. Furthermore, these findings have management implications with potentially additional intervention measures required to mitigate reduced riverbank stability during INNPs removal activities.